Cyanohydrins are key intermediates for synthesizing useful substances such as optically active α-hydroxycarboxylic acids, α-hydroxyaldehydes, β-aminoalcohols and the like. One typical method for synthesizing a cyanohydrin is asymmetric cyanation reaction of an aldehyde and a ketone. A generally used reacting agent is trimethylsilyl cyanide, which is easily available. For cyanosilylation, it has been studied to use a catalyst amount of chiral Lewis acid or Lewis base (see J. Chem. Soc. Chem. Commun., 1973, pp. 55-56, and Chem. Ber., 1973, 106, pp. 587-593), and a high enantioselectivity has been reported. Furthermore, the present inventors have already proposed using a lithium salt or the like as a catalyst for cyanosilylation (see Japanese Laid-Open Patent Publication No. 2006-219457).
However, the technologies described in J. Chem. Soc. Chem. Commun., 1973, pp. 55-56, and Chem. Ber., 1973, 106, pp. 587-593 mentioned above have problems in that a large amount of catalyst (the ratio represented by substrate/catalyst is 1 to 100) is necessary, the preparation of the catalyst is troublesome, the catalyst is unstable, an excessive amount of trialkylsilyl cyanide compound with respect to a carbonyl compound needs to be used, the post-reaction treatment is troublesome, and the like.
According to the technology described in Japanese Laid-Open Patent Publication No. 2006-219457, the catalyst is easy to prepare and is highly stable, but there is a problem that an optically active product having a high enantioselectivity cannot be obtained. Namely, this technology uses only an achiral salt such as LiCl or the like as the catalyst, and so the product is entirely racemic (i.e., 0% ee).